This invention relates to a device for determining the effectiveness of stimulation in an electrical heart stimulator.
In the practice of heart stimulation, one of the characteristic features of a stimulator is the length of its service life, that is the service life of the power source (typically a battery) which powers it. This length of time is directly linked to the power consumption of the stimulating system, a significant component of which is the energy released in the form of electrical stimulation applied to the heart muscle.
The significance of this aspect is particularly obvious in systems which are designed to be implanted in a patient""s body. A stimulus is effective (and in this case it is said that it has xe2x80x9ccapturedxe2x80x9d the heart) if its energy exceeds a minimum value, the so-called xe2x80x9cstimulation thresholdxe2x80x9d or xe2x80x9ccapture thresholdxe2x80x9d. This threshold value depends on the stimulating system and the characteristics of the heart muscle involved.
In particular, it cannot be assumed that the value of the stimulation threshold remains constant over time. Because in current practice the energy of the stimulus is decided upon and set by the cardiologist when the unit is checked, and cannot be altered until a subsequent check, the solution currently adopted is to set the energy of the stimulus at a value substantially higher than the stimulation threshold. This is in order to guarantee effective stimulation for different stimulation threshold conditions. A consequence of this is the fact that the energy delivered by the heart stimulator with every stimulus can be very much greater (even four times or more greater) than the minimum which is necessary and sufficient.
There is therefore in general a need to have systems such that energy can be saved when providing the stimulating action, while at the same time ensuring that the effectiveness of the stimulation is constant. This is in order to provide a significant advantage in the design of a heart stimulator, among other things providing a longer service life for the device.
As a rule, satisfaction of the requirement stated above requires that the stimulator be capable of establishing whether it has successfully induced contraction of the heart muscle when delivering a stimulus. With this information the system can establish the value of the stimulation threshold sufficiently frequently (and even for each individual stimulus) and adjust the energy of the stimulus to minimize the proportion of energy which is actually wasted.
In general terms, the stimulus delivery system can be regarded as an electrical circuit comprising the stimulator itself, the electrode which delivers the stimulus to the heart and the complex of physiological tissues which returns the stimulus current to the stimulator: the area of heart muscle in contact with the terminal of the stimulation electrode constitutes the xe2x80x9cactivexe2x80x9d part of the electric circuit.
The behavior of this circuit has special features which are generally known and do not therefore need to be referred to in detail here. This is apart from one aspect, which is linked to the fact that once the stimulusxe2x80x94comprising a short electrical pulse of the magnitude of the order of a few volts and lasting of the order of a millisecondxe2x80x94has come to an end, part of its energy remains trapped in the circuit, giving rise to an appreciable potential difference which decreases over time as this energy is dissipated until the entire system returns to its initial conditions over a period of a few hundred milliseconds.
This tail electrical potential, usually known as the post-potential or stimulation artifact, or again the electrode polarization potential, may have a magnitudexe2x80x94measured immediately after stimulationxe2x80x94which is still of the order of a hundred millivolts. A typical profile for a post-potential signal of the type described is illustrated in profile a) of FIG. 1.
On the other hand, in addition to mechanical contraction of the muscle, the heart""s response to an effective stimulus is also manifested by an electrical response, known as the evoked potential, which is linked to the electrical activity of the cells during the contraction stage. This electrical potential (having the characteristics of a pulse of varying shape, lasting a few tens of milliseconds and of a magnitude of a few millivolts, which typically arises 10 to 50 milliseconds after the stimulus) can also be observed in the stimulator circuit, but superimposed on the stimulation post-potential. The magnitude of the latter may however be such as to render identification of the evoked response in the heart difficult.
A typical profile of an evoked response signal is shown in the bottom diagram, indicated by b) in FIG. 1. It will be appreciated that the two diagrams a) and b) in FIG. 1 are not to scale and that typically the peak for the post-potential signal may correspond to a value 10 to 100 times greater than the peak value for the evoked response signal. The waveform which can be observed after each effective stimulus is the result of the overlap (algebraic sum) of the two waveforms illustrated. If the stimulus is not effective, the component due to the evoked response (diagram b) will obviously be absent.
The complexity of problems described above has already been considered by the art through the adoption of a variety of solutions. There are systems in which detection of the evoked response is based on an analog filtering process with amplification of the potential measured on the stimulating electrode in comparison with a reference potential. Solutions of this type are described in for example documents EP-A-0 717 646, U.S. Pat. No. 5,561,529, U.S. Pat. No. 5,443,485, U.S. Pat. No. 5,718,720 and U.S. Pat. No. 5,873,898.
In substance these solutions provide for the greatest possible amplification of the evoked response and attempt to suppress the undesired part due to the stimulus post-potential as much as possible (typically through filtering).
This process has however proved difficult because, in the first place, as has been seen, the signal corresponding to the stimulus post-potential usually has a magnitude which is very much greater than the signal corresponding to the heart""s evoked response, and the frequency spectra of the two signals in question largely overlap and therefore cannot be separated by filtering in the frequency field.
In particular an amplification and linear filtering system can easily be saturated by the post-potential signal, thus making it impossible to detect any evoked response by the heart.
The functioning of other systems is based on the presence or absence of events which are indirectly linked with capture, such as e.g., the occurrence of spontaneous heart contractions before and after the stimulus which are detected by methods which are well-known in the art of heart stimulation (see for example documents EP-A-0 850 662 and U.S. Pat. No. 5,861,012).
Of the methods based on knowledge of past events, some operate by comparing the profile of the potential after the stimulus with a sample signal in which only the post-potential is present without the evoked response. In order to establish that the heart muscle has been captured in a generic stimulus, the corresponding signal is compared with the sample signal, and capture is therefore stated to have occurred when the differences with respect to the sample are sufficiently large.
Solutions of this type are described in documents U.S. Pat. No. 4,674,508, , U.S. Pat. No. 4,686,988, U.S. Pat. No. 4,729,376, U.S. Pat. No. 4,817,605, U.S. Pat. No. 5,350,410, and U.S. Pat. No.5,417,718.
These systems have two main disadvantages. First, in order to obtain a sample signal, it is necessary to perform a specific operation comprising the release of a stimulus which is reliably ineffective (there are various techniques for achieving this result) followed by recording of the response generated. Second, the form and amplitude of the stimulation artifact can change, and in fact change in relation to the energy of the stimulus. Thus, the operation described in the preceding paragraph must theoretically be performed whenever the characteristics of the stimulus are changed. These disadvantages make the above-mentioned systems more complex to construct, for equal effectiveness.
Yet other systems attempt to improve the discernibility of the heart response by reducing the magnitude of the post-potential or stimulation artifact as much as possible. These systems nevertheless require the use of special electrodes in which the phenomenon of the post-stimulation potential is minimized. These systems attempt to compensate for the stimulation post-potential by injecting into the circuit an amount of electrical energy identical and contrary to that which is expected as a residue. Examples of solutions of this type are found in documents U.S. Pat. No. 4,373,531, , U.S. Pat. No. 4,399,818, U.S. Pat. No. 4,821,724, U.S. Pat. No. 5,172,690, U.S. Pat. No. 5,741,312 and U.S. Pat. No. 5,843,136.
In particular, systems based on post-stimulus compensation have proved to be subject to appreciable criticality. Even a small error in estimation of the energy required is in fact sufficient to make it difficult to distinguish the evoked response. Furthermore, the use of special stimulating electrodes (typically of the type known as xe2x80x9csteroid elutingxe2x80x9d) constitutes a constraint which is not always accepted in the practice of heart-stimulating implants.
Of the systems described above, some function intrinsically through observing a series of successive stimuli, which makes it impossible to detect capture stimulus by stimulus. In this respect reference may be made, for example, to documents EP-A-0 765 177, U.S. Pat. No. 4,674,508, U.S. Pat. No. 4,729,376, U.S. Pat. No. 4,817,605, U.S. Pat. No. 5,741,312, U.S. Pat. No. 5,476,487 and U.S. Pat. No. 5,411,533. Yet other systems depend on the use of bipolar electrodes, which imposes a constraint upon their use. Examples of this type are documented in EP-A-0 561 781, U.S. Pat. No. 3,949,758, U.S. Pat. No. 4,817,605, U.S. Pat. No. 4,878,497, U.S. Pat. No. 5,265,603 and U.S. Pat. No. 5,324,310.
It will be appreciated that some of the documents provided as examples of various categories of solutions considered above have been cited more than once. This is due to the fact that in various cases a document constitutes an example of more than one of the solutions considered from time to time.
This invention therefore has the purpose of providing a heart stimulating system capable of simultaneously satisfying one or more of the following requirements:
1) the possibility of using it regardless of the type of electrode available, whether of the single pole or bipolar type, avoiding the need to use electrodes having special characteristics and/or of a special type,
2) the possibility of doing away with the acquisition of a reference sample,
3) limiting observation to the events which occur immediately after stimulation, without the need to observe other indirect events (e.g., spontaneous sensing etc.),
4) the possibility of avoiding methods of processing the stimulus post-potential (either by filtering means or using electrical compensation methods) in order to eliminate it or reduce its magnitude, and
5) the possibility of deciding on the relative effectiveness, stimulus by stimulus, without the need to perform a statistical observation of a number of consecutive stimuli.
The solution according to the invention is based on a circuit capable of tracking the artifact of stimulation while avoiding saturation of the amplification stage, so as to be able to convert the signal detected at the electrode into a series of electrical pulses whose sequence in time reproduces the profile of the potential (post-potential plus any evoked response). Processing of the above-mentioned pulses, based on an algorithm used by a processing unit located on board the stimulator, and therefore capable of being implanted, makes it possible to establish whether the heart has been captured by the stimulus, reliably and with certainty.
This invention is a device for determining the effectiveness of electrical stimulation of heart muscle from a signal comprising a post-potential component having, in the event of effective stimulation, a superimposed evoked response component. The device comprises a differential stage with a first input for application of the signal and a second input for application of a feedback signal, the differential stage generating a corresponding output signal whose level is determined by the levels of the signals present at the first and said second inputs; and a comparator stage including a feedback unit configured to act on the second input in a follower relationship to the signal present at the first input avoiding saturation of the differential stage; the feedback unit being configured to generate at least one compensating signal indicative of the variation of the signal present at the first input over time, the at least one compensating signal being indicative of the presence of the evoked response. The comparator stage may comprise at least two threshold levels which, when reached by the output signal from the differential stage, are indicative of possible saturation of the differential stage relative to its linear functioning dynamics. The comparator stage also may comprise at least one further threshold defining a field of values for the output signal from the differential stage in which the feedback unit is substantially inactive. The feedback unit may be configured to generate first and second compensation signals which are indicative, respectively, of the divergence between the output signal from the differential stage in a first and a second direction, respectively, with respect to a selected reference level. The feedback unit also may be configured to apply a signal obtained from the sum of the first and the second compensation signals to the second input of the differential stage. The sum may have different signs. The feedback unit may have an integrator stage configured to generate a signal applied to the second input by integration.
The compensating signal may be a pulsed signal in which the frequency of the pulses is indicative of the difference between the output signal from the differential stage and a selected reference level. The comparator stage may be configured to generate first and second compensation signals of the pulsed type, in which the pulse frequency is indicative of the derivative of the signal present at the first input with respect to time, the first and second compensation signals being generated alternately between them according to the sign of the derivative.
The device may also include a counter, wherein the first and second compensation signals are input respectively as increasing and decreasing signals to the counter, the progression of the count in the counter over a selected period of time comprising a sequence of count signal values indicative of the effectiveness of the stimulation pulse. The counter may be configured so as to be zeroed corresponding to the action of electrically stimulating the heart muscle and may be enabled after a predetermined time interval following stimulation of the heart muscle. The counter may be enabled for the purposes of performing the corresponding count during a time window of a size determined from the stimulating effect. The time window may range from 50 to 60 milliseconds.
The device may further comprise a processing logic module which is capable of applying to the count signal values from the counter during the selected period of time at least one criteria for identifying the effectiveness of stimulation selected from the group of:
1) whether the sum of all the negative values which are greater in absolute value than a selected threshold value exceeds a predetermined limit,
2) whether the maximum value of the count signal values, in modulus and sign, is greater than the first value of the count signal values incremented by a specified amount, and
3) whether reduction of the sequence of the count signal values by interpolation into a series of segments of a straight line identified by their corresponding angular coefficients, with subsequent comparison of the angular coefficients with corresponding pairs of selected limit values results in at least one of the angular coefficients exceeding the corresponding pair of limit values.
The processing logic module may be configured to generate an output signal indicative of effective stimulation when an affirmative result is obtained from one of the criteria, to apply the criteria in sequence, passing on to the next criterion if a negative result is obtained from one of the criteria, or to generate an output signal indicative of ineffective stimulation when all three of the criteria yield a negative outcome. The device may further include means for detecting excursion of the count signal values during the selective period of time between a selected maximum and a selected minimum value and for declaring that the stimulating action is ineffective if the detected excursion is less than a predetermined limit. The processing logic module then may be configured to detect a difference between a maximum value and a minimum value of the sequence of count signal values subjected to the third criterion and to apply the third criterion only if the difference between the maximum value and the minimum value is greater than a predetermined limit. The processing logic module may be configured to avoid application of the third criterion when the difference between the maximum value and the minimum value is less than the predetermined limit.
The processing logic module may be selected from:
a filtering module to reduce the spectral content of the signal transferred to the logic module at higher frequencies,
a module to differentiate the sequence of values subjected to the filtering, and
a module to translate the values obtained from this differentiation in such a way that the last of them is always zero.